1. Field of the Invention
This invention relates to a hysteresis comparator, and more particularly to a hysteresis comparator with a small hysteresis width that allows two-terminal input.
2. Description of the Related Art
Published Examined Japanese Patent Application No. 53-17029 discloses a conventional hysteresis comparator characterized by having a small hysteresis width and allowing two-terminal input. The phrase "hysteresis width" is used in the specification to means a voltage difference between a value of an input voltage in which a state of an output voltage is inverted and another value of the input voltage in which another state of the output voltage is inverted. Such a hysteresis comparator has problems with the stability of hysteresis width and the operating regions of circuit elements. These two problems will be explained briefly, referring to FIG. 1.
With the hysteresis-width stability problem, increasing the area ratio N.sub.1 of p-n-p transistors Q62 to Q61 and the area ratio N.sub.2 or p-n-p transistor Q72 to Q71 results in the change of the hysteresis width by the increased amount of base currents flowing through these p-n-p transistors Q61, Q62, Q71, and Q72. The same holds true even if those area ratios are relatively small, for example, N.sub.1 =2 and N.sub.2 =2. For IC fabrication, p-n-p transistors Q61, Q62, Q71, and Q72 must be fabricated in lateral structure, resulting in a small current gain h.sub.FE. The small current gain makes it difficult to control the collector current, leading to the unstable hysteresis width. If p-n-p transistors Q61, Q62, Q71, and Q72 are replaced with n-p-n transistors, this requires input transistors Q51 and Q52 to be lateral p-n-p transistors, which raises an input current problem.
With the operating region problem, the operation of p-n-p transistors Q62 and Q72 in the saturation region is not only undesirable for faster operation and but also liable to develop parasitic elements.
As mentioned above, while conventional hysteresis comparators feature two-terminal input and their small 1 hysteresis width, they have problems with the stability of hysteresis width and the operating regions of circuit elements.